What should I do if I put the wrong fuel in my car?

Putting petrol in a diesel car, or the other way around, is a surprisingly easy mistake to make.

According to the RAC, around 150,000 people accidentally put the wrong fuel in their car each year. From an over-tired parent reaching for the pump, to being distracted by another work email or simply forgetting to check, it’s no surprise that so many drivers are being added to this ever-growing fuel mix-up statistic.

Because diesel has a wider nozzle which is harder to fit into a petrol car, 95% of misfuelling happens when petrol is put into a diesel car. That’s what we’re going to focus on in this article.

Guide Contents:

How far can you drive with petrol in a diesel car?

What to do when you put petrol in a diesel car?

What are the symptoms of putting petrol in a diesel car?

What's the worst case scenario when you put petrol in a diesel car?

What's worse, diesel in a petrol car or petrol in a diesel car?

How to tell the difference between petrol and diesel

Is my car diesel or petrol?

How far can you drive with petrol in a diesel car?

The better question to ask would be should you drive with petrol in a diesel car, and the answer is a definite “no". As soon as you start the engine, the damage will begin, and it’s not just limited to one area. It isn’t possible to drive a diesel car using petrol. 

Your car might work for a short while, but after that you’ll need to pay for expensive repairs. The best thing to do is double check which pump you've grabbed before you start filling up.

What to do when you put petrol in a diesel car

Putting petrol in a diesel car can cause more damage than putting diesel in a petrol. If this does happen, don’t fret –  there is significant damage limitation if you realise you’ve used the wrong fuel before you start the engine. 

All you have to do is call your breakdown provider or mobile mechanic and they’ll get it sorted – just make sure to go to get your car checked over!

Then it's just a case of draining your tank and refilling it with the correct fuel. If you spot your mistake quickly, you should be okay, as it’s commonly agreed that you can mix up to 5% diesel in a petrol tank without breaking your car or the bank.

The real damage happens when you start the car and drive away. As soon as you turn the key in the ignition and the dashboard lights up, your fuel pump has come to life. The engine is supposed to be primed with diesel before it starts, but when you accidentally put petrol in it, it will be sucking the offending liquid into the fuel lines. So, not only will your tank then need to be drained, your fuel pipes will need to be flushed as well – which isn’t cheap!

What are the symptoms of putting petrol in a diesel car?

• Lack of throttle power despite pressing the accelerator.

• Thick black smoke.

• Engine cutting out.

• Engine starting to cough or chug.

• Problems starting/restarting the vehicle.

• An unusual smell.

What’s the worst case scenario when you put petrol in a diesel car?

As we mentioned earlier, sticking your key in the ignition is the worst thing you can do. If your engine does actually start, you’re in for some real trouble.

This is because a modern diesel engine utilises different types of technology to save mpg figures and minimise emissions. The fuel pumps are greased with diesel as it passes through.

When petrol enters these diesel pipes, rather than lubricating the tubes, its solvent properties create internal friction. This is because the metal surfaces grind together, which causes them to disintegrate, and create a swarf. Microscopic particles of metal could be created, causing further damage to your car’s engine.

After the petrol has gone through the pumps, the engine’s high-pressure injectors are the next thing to go. They force fuel through the engine’s cylinders via incredibly fine holes with an engineered spray pattern. If this swarf gets into the injection system, it will block some or all of the holes. This really is the worst case scenario as a common rail injector system could cost thousands of pounds to replace.

If you think you’ve put the wrong fuel in your car then find and call a local garage or breakdown service to get it fixed as soon as possible!

What’s worse, diesel in a petrol car or petrol in a diesel car?

Putting petrol in a diesel car is considerably worse than making the mistake the other way around. 

If you put diesel in a petrol car, the engine will run on lower power and there will be lots of smoke. The fuel can be drained from the tank and replaced without any long term problems.

On the other hand, running a diesel engine on petrol could cause thousands of pounds worth of damage. If you realise you’ve put petrol in your car incorrectly, don’t start the engine, push the car to a safe place and wait for assistance from a professional.

How to tell the difference between petrol and diesel

Petrol and diesel both have distinct smells which can be used to identify them. The latter fuel is a lot thicker and black in colour, whereas petrol can be light red and light green. 

Similarly, petrol fuel pumps are colour coordinated, with black for diesel and green for petrol. 

Is my car diesel or petrol?

If you need to work out what type of fuel a particular car needs, the fuel cap label should indicate this. You can also tell by sound; diesel engines make a louder, harsher noise compared to the generally smoother sound of a petrol car.